The double layer concept was first described by Helmholtz in the last half of the nineteenth century and has been modified by many other since. In essence, an electrical double layer occurs at any phase boundary and consists generally of an immobile layer of ions next to the surface of one phase and a mobile layer of ions electrostatically in equilibrium with the ions in the immobile layer. Excellent reviews of double layer phenomena can be found in Smoluchowski, Handbuch Der Elektrizitat und des Magnetismns, Volume II, pp. 336-348, Lograetz, Editor, Leipsig, Barth, 1921 and Boumans, Physica 23, 1007-1026 (1957).
Furthermore, a more general description of the double layer can be found in two related patent applications by a different inventive entity, but including one of the present inventors. These applications, incorporated herein by reference in their entirety, are Findl et al Ser. No. 64,600 filed Aug. 7, 1979, now U.S. Pat. No. 4,254,377, and Findl et al Ser. No. 64,601 filed Aug. 7, 1979, now U.S. Pat. No. 4,274,937.
The discoveries that were reported in the above patent applications were made during some basic research programs. Further research was done in this area in response to the need for on-line process monitoring and optimization that exists in industry. More particularly, as supplies of high grade ores diminish, a greater emphasis will be placed on a process for mineral enrichment. In light of this, it is surprising to find that little on-line equipment exists.
The theory of the double layer has practical application whenever it is desired to measure changes in the surface charge of a solid in suspension in a liquid. One theory is that the solid can be taken out of suspension whenever the surface electrical properties at the solid liquid interface is favorable. Those processes are believed to include water purification, decantation, flocculation, centrifugation, calrification, thickening, froth floatation using microbubbles, electrofloatation, ultrafiltration, microfiltration, expression, electrophoresis, electrodialysis, electrodecantation, and electrochromatography. As a specific example, various floatation processes are commonly used throughout the country to upgrade low quality iron ores before smelting. However, as mentioned above, no on-line monitoring process is presently feasible so that the surface charge environment can be kept at an optimum state.
All known present on-line monitoring methods are slow, complex and are essentially batch approaches. There is, therefore, a need in the industry for a real-time, continuous measure of changes in the surface charge using a simple, inexpensive, accurate and compact sensor and using a simple method.
The prior art also contains numerous sensors which, with modification, could be used to measure effects on the double layer. However, presently available potentiometric electrochemical sensors, be they wire electrodes, ion specific electrodes, isfets or enzyme electrodes, are all used in conjunction with reference electrodes that present an interface of the test solution. Thus, these conventional electrodes are subject to signal interpretation errors, or interfaces that make them unsuitable for surface charge measurement.